We analysed egg-hatching rates, the resulting clutch size and fledging weights in relation to prey abundance in Great Tits Parus major in a mixed deciduous forest in Switzerland. We show that the number of oaks within 20 m from the nest affected both clutch size and fledging weight positively. During a cold spell females delaying the hatching of their clutch incurred a cost in terms of reduced hatching success. On the other hand they readjusted the nestling period of their brood to the delayed caterpillar peak. Thus, fledging weights were higher compared to non-delayed broods. Since fledging mass is a decisive determinant of post-fledging survival and recruitment, we conclude that delaying the hatching of the brood when caterpillars are late yields a net gain in fitness and thus is a strategy rather than the consequence of energy constraints during the laying and incubation period.